A french threaded bottom bracket is a good start for that assumption. Also, I just took the locknut off the headset, and the steerer dimension indicate it's french threaded as well.

Three things make me think it is not French

27.2mm seatpost

22.2 stem

Seat lug threaded

It's a Raleigh from the Worksop facility. W for Worksop, K for the 11th fortnight, 8 for year of manufacture (1978). The cutouts on the BB are another clue as are the cutouts on the lugs. I have the same cutouts on my 753 Team Pro. As to the French threading, it is not the first Raleigh I have seen with French BB. There was one on fleabay earlier this year. Threading could be specified by whoever ordered the frame along with other details. Based on the seatpost size, it is most likely 531.

Wow, that's a very unusual [and very nice] bike. The seatpost size would indicate it is definitely not standard French metric tubing, so this is a real surprise. ~ Of course, the seat tube could have been later reamed out which would make the tubing scary-thin, but would allow for a larger diameter seatpost. ... Have you measured the outer diameters of the main frame tubes? Even allowing for the paint thickness this would give a reasonable idea of the tube diameters.

And, the steering tube inner diameter is also a bit of a surprise... although I actually have seen some later 1970s steerers with larger inner diameters than the narrower French thread headsets would suggest. The Campy braze-on cable guides over the bottom bracket, and #1010/B dropouts do suggest a very late 1970s or early 80s bike, but I would mistakenly have guessed it Italian made - and especially with the Campy #621 style chainstay cable stop. But, since it has a right threaded 35 x 1 French (versus right threaded 36 x 24 obsolete Italian) fixed cup on the bottom bracket that would blow away this idea completely.

Whatever it is, that looks like a very high quality racing frame. There were so many different builders and so many odd combinations of threads and sizes and components used as the French industry transitioned from French standards to later ISO in the early 80s, that French bikes are always a challenge to identify
I'll be very anxious to discover if anyone can come up with a final ID for you. _ Great find!

Wow, that's a very unusual [and very nice] bike. The seatpost size would indicate it is definitely not standard French metric tubing, so this is a real surprise. ~ Of course, the seat tube could have been later reamed out which would make the tubing scary-thin, but would allow for a larger diameter seatpost. ... Have you measured the outer diameters of the main frame tubes? Even allowing for the paint thickness this would give a reasonable idea of the tube diameters.

And, the steering tube inner diameter is also a bit of a surprise... although I actually have seen some later 1970s steerers with larger inner diameters than the narrower French thread headsets would suggest. The Campy braze-on cable guides over the bottom bracket, and #1010/B dropouts do suggest a very late 1970s or early 80s bike, but I would mistakenly have guessed it Italian made - and especially with the Campy #621 style chainstay cable stop. But, since it has a right threaded 35 x 1 French (versus right threaded 36 x 24 obsolete Italian) fixed cup on the bottom bracket that would blow away this idea completely.

Whatever it is, that looks like a very high quality racing frame. There were so many different builders and so many odd combinations of threads and sizes and components used as the French industry transitioned from French standards to later ISO in the early 80s, that French bikes are always a challenge to identify
I'll be very anxious to discover if anyone can come up with a final ID for you. _ Great find!

I originally thought i might be italian as well, it certainly has that sort of style, but I was very thrown off by the threading, plus the previous (not original) owner bought it in France. When I originally posted about the English steerer, it was based on the cinelli stem that was in it--which is 22.0 as well! (I knew they existed, but figured it was 22.2, and that a French BB/English HS combo wouldn't be too unusual for a French bike during the transition in standards.)

But as you suggested (and should have thought of), I measured the tubes and they appear to be English based (although the paint's a little thick). And it's hard to argue with CV-6's serial decoding. I checked on CR for some sort of match, and although I didn't find one for the lugs, the 753 bike on the Raleigh page has the same curious seatstay cap that's larger than the tube itself... So looks like I have a real oddball of a bike!

I originally thought i might be italian as well, it certainly has that sort of style, but I was very thrown off by the threading, plus the previous (not original) owner bought it in France. When I originally posted about the English steerer, it was based on the cinelli stem that was in it--which is 22.0 as well! (I knew they existed, but figured it was 22.2, and that a French BB/English HS combo wouldn't be too unusual for a French bike during the transition in standards.)

But as you suggested (and should have thought of), I measured the tubes and they appear to be English based (although the paint's a little thick). And it's hard to argue with CV-6's serial decoding. I checked on CR for some sort of match, and although I didn't find one for the lugs, the 753 bike on the Raleigh page has the same curious seatstay cap that's larger than the tube itself... So looks like I have a real oddball of a bike!

Not so oddball. Worksop did custom work and they did whatever the customer asked within reason. The way the seat lug is handled on your frame is another hallmark (am I allowed to use that? ) of Raleigh shops. It is identical to the way Team Pros are done and a good many Professionals, as I found out not long ago on this forum. Don't lose that bolt...it will be hard to replace.